A nonaqueous electrolyte battery using titanium oxide in a negative electrode enables stable and rapid charge/discharge and has a longer life than batteries using a carbon negative electrode. However, titanium oxide has a higher potential (namely, nobler) with respect to metal lithium than carbonaceous materials. In addition, the titanium oxide has a low capacity per unit weight. Therefore, a battery using titanium oxide in the negative electrode has a low energy density.
Because the potential of titanium oxide is caused by a redox reaction between Ti3+ and Ti4+ when lithium is electrochemically inserted and released, it is electrochemically limited. It is substantially difficult to improve energy density by shifting the potential of the electrode to the lower side also from the fact that rapid charge and discharge can be stably carried out at a higher electrode potential as high as about 1.5 V.
The theoretical capacity of titanium oxide is about 165 mAh/g in the case of titanium oxide having an anatase structure and about 170 mAh/g in the case of lithium-titanium composite oxide having a spinel structure such as Li4Ti5O12. On the contrary, the theoretical capacity of a graphite material is 385 mAh/g or more. Titanium oxide has a significantly lower capacity density than a carbon negative electrode. This is due to a drop in substantial capacity because of a few sites to which lithium is inserted in the crystal structure of titanium oxide and lithium is easily stabilized in the structure.
In recent years, monoclinic titanium dioxide having a higher theoretical capacity than other titanium oxides has attracted remarkable attention (see, for example, R. Marchand, L. Brohan, M. Tournoux, Material Research Bulletin 15, 1129 [1980]). In monoclinic titanium dioxide, the number of lithium ions which can be inserted or released per one titanium ion is a maximum of 1.0. Therefore, monoclinic titanium oxide has a theoretical capacity as high as about 330 mAh/g.
For example, JP-A 2008-34368 discloses lithium ion storage battery using titanium oxide TiO2 having a bronze structure as a negative electrode active material. Further, JP-A 2008-117625 discloses a lithium secondary battery using, as an active material, titanium dioxide having a titanic acid bronze crystal structure.
However, when monoclinic titanium dioxide is used as an electrode material, there is a problem that a deterioration in the performance of the battery is significant and the life of the battery is shortened.